1. Field of the Invention
The present invention relates to an aluminum electrolytic capacitor, and in particular, relates to a radial lead aluminum electrolytic capacitor with/without gasket foil(s).
2. Description of Related Art
Common aluminum electrolytic capacitor generally comprises a lead wire, a cathode foil, an anode foil and a separator, wherein the lead wire is stitched to the corresponding cathode foil or the corresponding anode foil, and the separator is set between the cathode foil and the anode foil. In practical processing, the stitching places between the anode foil and the corresponding lead wire as well as the cathode foil and the corresponding lead wire are generally adopting stitching or cold welding, which easily cause burrs. And when the cathode foil, the anode foil and the separator are winded to form a wound unit, the burrs will perforate the separator, so that there is fire striking in the electrified common aluminum electrolytic capacitor or the common aluminum electrolytic capacitor in use, which leads to short circuit or explosion of the common aluminum electrolytic capacitor.
For overcoming the deficiency of the common aluminum electrolytic capacitor, an aluminum electrolytic capacitor with gasket foil is disclosed in China patent application CN 200520054439.9. The aluminum electrolytic capacitor comprises two lead wires and plurality of layers of the aluminum foils (anode and cathode foils) and separator, wherein the lead wires are stitched with the aluminum foils (anode and cathode foils), and the separator is set between two layers of the aluminum foils (anode and cathode foils). The lead wire and the surface of the aluminum foils (anode and cathode foil) are provided with gasket foils, one end of the gasket foil is stitched to the aluminum foil (anode and cathode foil), and the other end is in manner of freely extending (in other case, the gasket foil may be arranged only on the surface of the cathode foil). The gasket foil can be winded together with the aluminum foil (anode and cathode foil) and separator to form a wound unit. Compared with the common aluminum electrolytic capacitor, said aluminum electrolytic capacitor adopts gasket foil as a protection, to prevent the burrs of the aluminum foil from perforating the separator, and consequently to prevent short circuit and explosion of the common aluminum electrolytic capacitor by fire striking in the electrified common aluminum electrolytic capacitor or the common aluminum electrolytic capacitor in use; besides, compared with the common aluminum electrolytic capacitor, said aluminum electrolytic capacitor with gasket foil is resistant to high ripple current and has more times of charging/discharging. However, said aluminum electrolytic capacitor with gasket foil has the same deficiency as the common aluminum electrolytic capacitor has, which is not resistant to inverse voltage and high temperature.
Either the common aluminum electrolytic capacitor or said aluminum electrolytic capacitor with gasket foil must be processed with anode formed foil to form dielectric oxide film on the surface of the anode foil, so that the anode foil can be resistant to rated voltage. The anode formed foil is produced by processing aluminum product to original foil, and then etching the original foil on the surface to form etched foil to increase the surface area of the original foil, then oxidizing the etched foil to form a film of Al2O3 on the surface of the etched foil. The Al2O3 film of the anode foil is used as dielectric in producing the aluminum electrolytic capacitor.
Regarding the aluminum electrolytic capacitor, there are anode foil and cathode foil. Normally, cathode foil does not need forming processing, and its surface is natural oxide film, which has a certain voltage resistance (1-1.5V) in normal temperature. However, because of the inhomogeneity and instability of the film, the voltage resistance of the cathode foil can hardly be ensured, especially in high temperature. When the aluminum electrolytic capacitor is imposed by inverse voltage of its polarity, the water in the electrolyte will be electrolyzed into oxygen, which is be propitious to repair the oxide film of the cathode foil and enhance forming characteristics, because there is 1%˜5% water in the common electrolyte adopted by the present aluminum electrolytic capacitor. However, water easily interacts with aluminum (especially in high temperature), to bring hydration and form Al2O3 film on the surface of cathode foil, and the interaction will reduce the capacitance of the cathode foil, and decrease synthesis capacitance of anode and cathode foil, so that the capacitance of the capacitor decreases; besides, because of the hydrogen produced by the interaction of water and aluminum in the inner of the capacitor, the inner pressure of the capacitor increases. When the imposed inverse voltage increases, the temperature around the capacitor rises and the more gas is produced; meanwhile, the increased voltage and its surrounding temperature expand the case of the capacitor, sometimes loosen or even open security device (for example case vent), and there is possibility that the capacitor loses its function.
Similarly, since the surface of the gasket foil is natural film without further forming processing, there exists the same problem in the aluminum electrolytic capacitor with gasket foil.